Receiver transponder for protected networks

ABSTRACT

A receiver transponder that is implemented in an optical add/drop multiplexer (OADM) is disclosed. The OADM is used in short haul type networks and receives light signals from two opposite directions on input fibers ( 21, 23 ). The optical input signals are converted to electrical signals by optical-to-electrical (O/E) converters ( 51, 53 ). The output terminals of the converters are connected to an electronic switch ( 61 ), which provides protection switching in a protected ring type network. The output signal of the switch can be monitored ( 65 ) before the signal enters a reshaping circuit ( 67 ), where the signal is reshaped, filtered from a supervisory channel, and adjusted to a proper drive level for a laser ( 69 ). The optical signal from the laser can travel a significant distance through a fiber ( 71 ) to a client receiver or sustain other forms of attenuation and still have sufficient signal power for reliable detection. An electrical output signal can be provided ( 73 ) by the reshaping circuit. The converters can be used to protect for one another and to detect channel signal power and a supervisory channel at electric outputs ( 57, 59 ). The laser can be a low cost type since the laser is typically used for transmitting light over only moderate distances. The electric high frequency switch is generally more reliable and can be more easily monitored than a purely optical switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims benefit of priority to(i) Swedish Patent Application No. 9900991-2, filed Mar. 18, 1999, (ii)PCT Patent Application No. PCT/SE00/00544, filed Mar. 20, 2000, entitled“A RECEIVER TRANSPONDER FOR PROTECTED NETWORKS”, and (iii) U.S. patentapplication Ser. No. 09/637,027, filed Aug. 14, 2000, now issued as U.S.Pat. No. 6,639,703; all of which are incorporated herein by reference intheir entirety. The present application is also related to co-pendingU.S. patent application Ser. No. 09/936,961, filed Jan. 29, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical communication system, and ismore particularly related to a receiver transponder to be used in anoptical add/drop node.

2. Discussion of the Background

Network availability is of significant importance in today'stelecommunication systems and data communication networks. One way ofimproving the availability of such networks involves building protectionfeatures into the networks such that efficient means are provided toswitch traffic to a different path in the case of a link or componentfailure along the original path. With the rapid development of DWDM(Dense Wavelength Division Multiplexing) systems and research of generaland special techniques of building optical networks using, differentforms of WDM (Wavelength Division Multiplexing) systems generally, thereis a growing interest in developing an approach to handle protection inoptical transmission systems and optical networks.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical add/drop nodecomprising a receiver transponder which has switching capabilities witha high reliability and which can be easily monitored.

According to one aspect of the invention, a receiver transponder used inan optical add/drop node comprises a plurality of optoelectricconverters that are configured to convert received optical signals toelectric signals. A first one of the optoelectric converters isconnected to an optical fiber carrying light signals from a firstdirection. A second one of the optoelectric converters is connected toanother optical fiber carrying light signals from a second directionopposite the first direction. An electronic switch is coupled to theplurality of optoelectric converters and has a plurality of inputterminals, a signal output terminal, and a control input terminal. Eachof the first and second optoelectric converters has output terminalsthat are connected to the input terminals of the electronic switch. Thecontrol input terminal receives a signal to control the electronicswitch to select one of the input terminals from which one of theelectric signals is switched to the signal output terminal. The abovearrangement advantageously provides a cost effective optical networkprotection system.

According to another aspect of the invention, a protected networkcomprises a first optical fiber path carrying light signals in a firstdirection, and a second optical fiber path carry light signals in asecond direction that is opposite the first direction. A plurality ofoptical add/drop nodes are coupled to the first optical fiber path andthe second optical fiber path to form a ring configuration. Each of theoptical add/drop nodes includes a receiver transponder that has aplurality of optoelectric converters, which are configured to convertreceived optical signals to electric signals. A first one of theoptoelectric converters is connected to the first optical fiber path,and a second one of the optoelectric converter is connected to thesecond optical fiber. Each of the optical add/drop nodes also includesan electronic switch that is coupled to the plurality of optoelectricconverters. The electronic switch has a plurality of input terminals, asignal output terminal, and a control input terminal. Each of the firstand second optoelectric converters has output terminals that areconnected to the input terminals of the electronic switch. The controlinput terminal receives a signal to control the electronic switch toselect one of the input terminals from which one of the electric signalsis switched to the signal output terminal. Under this approach, areliable optical protection scheme is achieved.

According to yet another aspect of the invention, a receiver transponderused in an optical add/drop node comprises a plurality of convertermeans for converting received optical signals to electric signals. Afirst one of the converter means is connected to an optical fibercarrying light signals from a first direction. A second one of theconverter means is connected to another optical fiber carrying lightsignals from a second direction opposite the first direction. Aswitching means is coupled to the plurality of optoelectric convertersand has a plurality of input terminals, a signal output terminal, and acontrol input terminal. Each of the first and second converter means hasoutput terminals that are connected to the input terminals of theswitching means. The control input terminal receives a signal to controlthe switching means for selecting one of the input terminals from whichone of the electric signals is switched to the signal output terminal.The above arrangement advantageously provides a simple way to monitorsignals within an optical network with protection capability.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 a is a diagram of a network which can handle a single fault in anoptical fiber, in the cable holding a pair of fibers and connecting theOADMs or in an OADM;

FIG. 1 b is a diagram of a network similar to that of FIG. 1 a whichgives the same level of protection and allows a more efficient use oftransmitter power and a reuse of wavelengths in the network;

FIG. 1 c is a diagram of a network similar to that of FIG. 1 a whichgives a better level of protection and which can handle a single faultin a transmitter or in a transponder;

FIG. 2 is a block diagram of the receiving side using an optical switchin an optical add/drop node in the networks of FIGS. 1 a–1 c; and

FIG. 3 is diagram of a network using an electric switch in an opticaladd/drop node in a network as illustrated in any of FIGS. 1 a–1 c.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for the purpose of explanation, specificdetails are set forth in order to provide a thorough understanding ofthe invention. However, it will be apparent that the invention may bepracticed without these specific details. In some instances, well-knownstructures and devices are depicted in block diagram form in order toavoid unnecessarily obscuring the invention.

The present invention provides a receiver transponder that includesoptical-to-electrical (O/E) converters. The wavelength channels, whichare to be dropped in the node from the left and the right fiberdirections, are converted by their respective optical-to-electrical(O/E) converters. The output terminals of the O/E converters areconnected to an electronic high frequency (HP) switch, which providesprotection switching and can be implemented at a low cost and using veryreliable components, such as a FET attenuator in each arm. The outputsignal from the switch can be monitored to determine the quality of thereceived signal before the signal enters a reshaping circuit. Thereshaping circuit, in turn, outputs to a low cost laser. The signal isnow amplified and reshaped, and hence, is relatively insensitive to theconditions between the laser and the client equipment. Accordingly, thesignal from the laser can travel a significant distance through anoptical fiber to the client receiver or sustain other forms ofattenuation and still have a signal power that is sufficient forreliable detection.

FIGS. 1 a–1 c show diagrams of various optical networks that exhibitdifferent levels of ring, protection in the optical layer. As shown inthe figures, the networks all have a ring structure and contain OpticalAdd Drop Multiplexers (OADMs) 1, which are also referred to as opticaladd/drop nodes. The OADMs 1 contain the filters and couplers necessaryto add, drop, and block wavelengths that are terminated in the node 1.Each such OADM 1 is connected to a neighboring OADM through pairs ofoptical fibers 3, wherein one pair provides connectivity on a leftwardor western direction and a second pair connects in a rightward oreastern direction.

In the scheme of FIG. 1 a, each OADM 1 is connected to atransmitter-responder or transponder (TP) 5 and to a receiver (R) 7through an optical switch 9. The transponder 5 transmits the wavelengthsignal in both directions via a connector 8. The receiver 7 can receivedesired light signals by selecting via an optical switch 9 theappropriate port of the OADM 1. Upon receipt of the light signals, thereceiver 7, for example, converts such signals into electrical signals.

The network, as illustrated in FIG. 1 a, provides are two differentpaths for the transmission of information from one node to another node.The first path extends in a clockwise direction; and, the second pathextends in a counter-clockwise direction. A single inactive link existsbetween any two adjacent nodes. Although both the first and second pathscan be used simultaneously, under normal operating conditions, only oneof the two possible paths is used to carry traffic from one node toanother node. When a fault occurs in this path, the network diverts thetraffic to the other path (i.e., serving as a protection path), therebyproviding a protective feature. Such a protective scheme can handlesingle faults in an optical fiber within the cable holding the pair offibers that connects the OADMs 1 or within the OADMs 1.

Another protective scheme provides a single inactive link between anytwo adjacent nodes, while all other links are used for transmission oftraffic. The position of the inactive link can then be displaced when afault occurs.

The optical network of FIG. 1 b provides a similar level of protectionas that of FIG. 1 a; however, the configuration of FIG. 1 may allow amore efficient use of transmitter power and reuse of wavelengths in thering architecture. The transponders 7 are connected to the OADM 1through optical switches 11, which provide the capability to selectamong multiple ports of the OADM 1 as to direct the transmission of thelight signals in either the clockwise direction or counter-clockwisedirection.

The design of a protective network needs to consider the reliability ofthe transmitter optical switches 11 as well as the possibility ofmonitor the status of the protection path. In the optical network ofFIG. 1 c separate transponders 5′, 5″ are provided for transmitting ineach direction. Accordingly, transmitter optical switches 11 (FIG. 1 b)are not needed. In this third network scheme, faults in a transmitter orin a transponder can be mitigated.

A common feature of each of the above schemes is that a switchingfunction (e.g., switch 9) on the receiving end is required to select thedirection from which the wavelength signal is to be received.Traditionally, a simple optical space switch can be deployed to providethis switching function, which provides an efficient solution forlong-haul WDM systems. The system architectures of these long-haulsystems are typically based on utilizing optical amplifiers as thefundamental building blocks and employing a separate wavelength channelfor supervisory signaling.

By contrast, metropolitan and wide area networks are short haul typeapplications, thus, other more cost effective systems and technologiesneed to be developed, while retaining necessary functionalities of thelong-haul solutions. As a consequence, the short haul solutions wouldnot necessarily be based on optical amplifiers; accordingly, it isimperative to minimize the attenuation between all ports in the node.Additionally, it is important to take into account all the networkfunctions that need to be implemented in connection with the opticalswitch (e.g., the switch 9 in FIGS. 1 a–1 c).

FIG. 2 shows a block diagram of the receiving side using an opticalswitch in an optical add/drop node in the networks of FIGS. 1 a–1 c. Thewavelength channels from other OADMs arrive at a particular OADM througha left input fiber 21 and a right input fiber 23, as illustrated in FIG.2. From these signals, a portion of the optical power is extracted usingoptical tapping couplers 25, 27 connected to the respective input fiber.The extracted signals are fed to optical-to-electrical converters 29,31, which convert the optical signals to equivalent electric signals.These converters 29, 31 can be designed to exhibit the appropriatesensitivity and dynamic range for the particular application.Additionally, converters 29, 31 can serve as back-up for each other inthe event of a failure in one of them; further, detection of channelsignal power and a supervisory channel can be readily performed at theseconverters 29, 31 with minimal cost.

The average power or the power levels of the two wavelength channels canthen be measured at the outputs 33, 35 of the converters 29, 31,respectively. Also, an overlaid embedded supervisory data channel can bedetected in the electric signals by feeding the detected instant powersignal to a supervisory channel receiver 37; it should be noted thatmultiple supervisory channel receivers 37 may be used. The detectedpower levels at the outputs 33, 35 are used to monitor the status of thepaths from the left and the right directions, respectively, and tochange the switch position of the optical switch 39 (FIGS. 1 a–1 c).

Because a separate supervisory wavelength channel entails significantcosts, both in terms of component cost and additional attenuation in thenode, an embedded channel solution is preferable. As shown, the otheroutput ports of the tapping couplers 25, 27 are connected to the opticalswitch 39. The position of the switch 39 determines the direction fromwhich the wavelength channel is to be received. The output of thisswitch 39 is fed into another optical tapping coupler 41, which has oneoutput connected to another optical-to-electrical converter 43 thatoutputs an electric signal at an output 45, from which the average powerof power level at the output of the optical switch 39 can be detectedand monitored. By comparing the power levels of the electric signals atthe outputs 33, 35, and 45 of the power detectors 29, 31, and 43, thestatus and the attenuation of the switch 39 can be derived. Anotheroutput 46 from the tapping coupler 41 is connected to the clientreceiver (the receiver 7 in FIGS. 1 a–1 c).

In the implementation of the network in FIG. 2, the following importantissues need to be considered: switch reliability, switch monitoringcapability, attenuation associated with the optical couplers and opticalswitches, cost of components, and management of the supervisory channel.Switch 39 constitutes a single point of failure in the link, and thus,the reliability of the switching components is critical. Unfortunately,it is difficult to test the long-term reliability of an optical switch;for example, many of the optical switches that are available on themarket do not have long-term reliability data. A related concerninvolves monitoring the optical switch to easily determine whether theoptical switch is performing properly.

Further, other issues, which may indirectly have an impact on the choiceof implementation, include equipment capability. For instance, thereceivers in the client equipment may be unsuitable for directlyreceiving a wavelength channel from an optical DWDM network. This may becaused by poor receiver sensitivity, dynamic range problems, and anincompatable detector. That is, the receiver incorporates a detectorthat cannot handle the particular wavelength.

In FIG. 3 the receiving part of an optical add/drop node is shown. Theoptical signals enter the node 1 at an input left fiber 21 and an inputright fiber 23. The left input fiber 21 is connected to a leftoptical-to-electrical or opto-electronic (O/E) converter 51, and theright input fiber is connected to a right opto-electronic (O/E)converter 53. Converters 51, 53 convert the incoming light signals toelectrical signals, for example, by sensing the instantaneous lightpower of the incoming signals and representing the sensed power with anelectric signal. The O/E converters 51, 53 can be designed to exhibitany suitable sensitivity and dynamic range for correctly converting thereceived light signals. In addition, each O/E converter 51, 53 hasoutput terminals 57, 59, which provide the electric signals thatrepresent the detected instantaneous channel signal power, from whichthe average power and a signal carrying a supervisory channel can bedetected by monitoring circuits (not shown).

The main output terminals of the O/E converters 51, 53 are connected toan electronic high frequency (HF) switch 61, which is controlled by aswitch control signal that is input on at a control input terminal 63.The HF switch 61 performs the protection switching and can bemanufactured at a low cost using very reliable components, such as FETs(Field Effect Transistors). A portion of the output signal of the switch61 is provided to monitoring circuits (not shown) via electric line 65to determine the quality of the received signal. The other portion ofthe electric output signal is provided to a reshaping circuit block 67in which the signal is reshaped, filtered from a supervisory channel,and adjusted to a proper power level en-route to a laser 69. The laser69 can be

A low cost laser, which can be use for many applications, is used forlaser 69. The optical signal output from the laser 69 can travel asignificant distance through a fiber 71 to a client receiver (not shown)or sustain other forms of attenuation and still have a sufficient signalpower for reliable detection. If an electrical output signal isdesirable, such electrical signal is provided at the output of thereshaping circuit 67, as represented by the electric line 73. From suchan electrical output signal, a signal can be extracted for performancemonitoring of client channels.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A receiver transponder used in an optical add/drop node, the receivertransponder comprising: a plurality of optoelectric convertersconfigured to convert received optical signals to electric signals, afirst one of the optoelectric converters being connected to an opticalfiber carrying light signals from a first direction, a second one of theoptoelectric converters being connected to another optical fibercarrying light signals from a second direction opposite the firstdirection, each optoelectric converter generating an output signalindicative of power of the received optical signals; an electronicswitch being coupled to the plurality of optoelectric converters andhaving a plurality of input terminals, a signal output terminal, and acontrol input terminal, each of the first and second optoelectricconverters having output terminals connected to the input terminals ofthe electronic switch, the control input terminal receiving a signal tocontrol the electronic switch to select one of the input terminals fromwhich one of the electric signals is switched to the control signalresponsive to the output signals indicative of the power of the receivedoptical signals output terminal; and a circuit connected to the outputterminal of the electronic switch that adjusts the signal output fromthe electronic switch to a predetermined power.
 2. A receivertransponder used in an optical add/drop node, said receiver transpondercomprising: a plurality of optoelectric converters configured to convertreceived optical signals to electric signals, a first one of theoptoelectric converters being connected to an optical fiber carryinglight signals from a first direction, a second one of the optoelectricconverters being connected to another optical fiber carrying lightsignals from a second direction opposite the first direction; anelectronic switch being coupled to the plurality of optoelectricconverters and having a plurality of input terminals, a signal outputterminal, and a control input terminal, each of the first and secondoptoelectric converters having output terminals connected to the inputterminals of the electronic switch, the control input terminal receivinga signal to control the electronic switch, the control input terminalreceiving a signal to control the electronic switch to select one of theinput terminals from which one of the electric signals is switched tothe signal output terminal; and a laser having an input terminal andbeing configured to produce a light signal responsive to output of theelectronic switch.
 3. A protected network comprising: a first opticalfiber path carrying light signals in a first direction; a second opticalfiber path carrying light signals in a second direction that is oppositethe first direction; and a plurality of optical add/drop nodes coupledto the first optical fiber path and the second optical fiber path toform a ring configuration, each of the optical add/drop nodes comprisinga receiver transponder comprising: a plurality of optoelectricconverters configured to convert received optical signals to electricsignals, a first one of the optoelectric converters being connected tothe first optical fiber path, a second one of the optoelectric converterbeing connected to the second optical fiber, each optoelectric convertergenerating an output signal indicative of power of the received opticalsignals; an electronic switch being coupled to the plurality ofoptoelectric converters and having a plurality of input terminals, asignal output terminal, and a control input terminal, each of the firstand second optoelectric converters having output terminals connected tothe input terminals of the electronic switch, the control input terminalreceiving a signal to control the electronic switch to select one of theinput terminals from which one of the electric signals is switched tothe control signal responsive to the output signals indicative of thepower of the received optical signals output terminal; a circuitconfigured to adjust the signal output from the electronic switch to apredetermined power.
 4. A protected network comprising: a first opticalfiber path carrying light signals in a first direction; a second opticalfiber path carrying light signals in a second direction that is oppositethe first direction; and a plurality of optical add/drop nodes coupledto the first optical fiber path and the second optical fiber path toform a ring configuration, each of the optical add/drop nodes comprisinga receiver transponder comprising: a plurality of optoelectricconverters configured to convert received optical signals to electricsignals, a first one of the optoelectric converters being connected tothe first optical fiber path, a second one of the optoelectric converterbeing connected to the second optical fiber, and an electronic switchbeing coupled to the plurality of optoelectric converters and having aplurality of input terminals, a signal output terminal, and a controlinput terminal, each of the first and second optoelectric convertershaving output terminals connected to the input terminals of theelectronic switch, the control input terminal receiving a signal tocontrol the electronic switch to select one of the input terminals formwhich one of the electric signals is switched to the signal outputterminal wherein the receiver transponder further comprises: a laserhaving an input terminal and being configured to produce a light signalresponsive to output of the electronic switch.
 5. A receiver transponderused in an optical add/drop node, said receiver transponder comprising:a plurality of converter means for converting received optical signalsto electric signals, a first one of the converter means being connectedto an optical fiber carrying light signals from a first direction, asecond one of the converter means being connected to another opticalfiber carrying light signals from a second direction opposite the firstdirection, each optoelectric converter generating an output signalindicative of power of the received optical signals; a switching meanscoupled to the plurality of optoelectric converters and having aplurality of input terminals, a signal output terminal, and a controlinput terminal, each of the first and second converter means havingoutput terminals connected to the input terminals of the switchingmeans, the control input terminal receiving a control signal responsiveto the output signals indicative of the power of the received opticalsignals to control the switching means for selecting one of the inputterminals from which one of the electric signals is switched to thesignal output terminal; and means for adjusting the signal output fromthe switching means to a predetermined power.
 6. A receiver transponderused in an optical add/drop node, said receiver transponder comprising:a plurality of converter means for converting received optical signalsto electric signals, a first one of the converter means being connectedto an optical fiber carrying light signals from a first direction, asecond one of the converter means being connected to another opticalfiber carrying light signals from a second direction opposite the firstdirection; a switching means coupled to the plurality of optoelectricconverters and having a plurality of input terminals, a signal outputterminal, and a control input terminal, each of the first and secondconverter means having output terminals connected to the input terminalsof the switching means, the control input terminal receiving a signal tocontrol the switching means for selecting one of the input terminalsfrom which one of the electric signals is switched to the signal outputterminal; and means for producing a light signal responsive to output ofthe switching means.
 7. The receiver transponder according to claim 6further comprising: means for adjusting the signal output from theelectronic switch to a predetermined power.